1. Field of the Invention
The present invention relates to an electron emitting device applicable to, for example, a display and a light exposure apparatus and a method of manufacturing the same, particularly, to a cold cathode type electron emitting device having a planar structure and a method of manufacturing the same.
2. Description of the Related Art
In recent years, a cold cathode type electron emitting device having a planar structure has been proposed. The device of this type, which is called a surface conduction device or a planar MIM device, includes a pair of electrodes arranged a prescribed distance apart from each other on a flat insulating substrate, a pair of conductive films formed between these electrodes, and an electron emitting layer formed on these conductive films. The particular electron emitting device, which has a simple structure as described above, is adapted for formation of an electron source array in which a large number of electron emitting devices are arranged on a single substrate.
As an application of the electron source array, a thin planar display has attracted attention. In the thin planar display, the phosphor is excited by electron so as to emit light as in a CRT. Since the energy efficiency of light emission based on the particular principle is high, it is possible to realize a spontaneous light emission type thin planar display achieving a low power consumption and exhibiting a high brightness and a high contrast by using the electron source array noted above.
An example of the planar MIM device is reported by, for instance, Bischoff et al. in “Int. J. Electronics, 1992, VOL. 73, NO. 5, 1009-1010” and “Int. J. Electronics, 1991, VOL. 70, NO. 3, 491-498”. FIG. 1 is an oblique view schematically showing the construction of the device reported by Bischoff et al. The planar MIM device shown in FIG. 1 includes a pair of metal electrodes 101a and 101b formed on an insulating substrate 100, a metal film 102 providing a micro-slit between the electrodes 101a and 101b, and a deposited film 103 formed at the position of the micro-slit of the metal film 102. The reference numeral 105 shown in FIG. 1 denotes the width of the micro-slit formed in the metal film 102. The width 105 is about 0.1 μm to 10 μm.
The device having the construction described above is prepared as follows. First, a pair of planar metal electrodes 101a and 101b are formed on the insulating substrate 100. Then, the metal film 102, which is sufficiently thin compared with the electrodes 101a and 101b and is thick enough to achieve electric conduction, is formed between the electrodes 101a and 101b. Further, an electric current is allowed to flow through the electrodes 101a and 101b so as to generate Joule heat in the metal film 102. As a result, the metal film 102 is partially melted and ruptured so as to be made discontinuous. In other words, a micro-slit is formed in the metal film 102. Incidentally, the resistance between the electrodes 101a and 101b is high immediately after the conductive film is rendered discontinuous. The treatment for rendering the conductive film discontinuous by the flow of current through the conductive film is called “B-forming (Basic forming)”.
Further, the resultant structure is subjected to a treatment called “A-forming (Adsorption-assisted forming)”. In A-forming, a voltage not higher than 20 V is applied between the electrodes 101a and 101b in a vacuum containing hydrocarbons. As a result, the resistance between the electrodes 101a and 101b is lowered over several minutes after application of the voltage, with the result that the current flowing between the electrodes 101a and 101b is increased.
On the other hand, Pagina et al. report in, for example, “Int. J. Electronics, 1990, VOL. 69, NO. 1, 25-32” that the entire region between the electrodes 101a and 101b after the A-forming treatment is covered with a conductive film, and that the conductive film is a thin film containing carbon.
Also, Bischoff et al. report in the publications referred to previously that the light emission is observed in addition to the electron emission by supplying an electric current into the device after the A-forming treatment. It is estimated by Bischoff et al., by the analysis of the emission spectrum, that it is necessary for the material constituting the deposited film 103 to be capable of containing thermoelectrons having a temperature up to 4,000 K and for the particular material itself to be capable of being heated to temperatures exceeding 1,000 K. Such being the situation, Bischoff et al. argue that the conductive film covering the region between the electrodes 101a and 101b after the A-forming, i.e., the deposited film 103, is a graphitized carbon film.
Incidentally, the deposited film 103 is electrically divided into small regions by a single or a plurality of boundaries. The width of the boundary is not larger than the tip of the probe of a scanning tunneling microscope, i.e., not larger than scores of nanometers. Concerning the detailed construction of the boundary portion, it is pointed out by Bischoff et al. in the publications referred to previously that the boundary portion is formed of slits each having a width of scores of nanometers. On the other hand, it is pointed out by Pagina et al. in the publication referred to previously that the edge portions of two carbon-like films overlap each other in the boundary portion. However, the detailed construction of the boundary portion has not yet been clarified sufficiently.
Concerning the current-voltage characteristics, the planar MIM device described above exhibits a VCNR (Voltage Controlled Negative Resistance) characteristics as shown in FIG. 2. Also, concerning the planar MIM device, it is reported by Pagina et al. in “Phys. Stat. Sol. (a) 108, 11(1988)” that the emission efficiency represented by the ratio of the emission current to the current flowing into the device, i.e., the device current, is very small, which is about 10−6.
The surface conduction device resembles the planar MIM device in construction. An example of the surface conduction device is reported in, for instance, Jpn. Pat. Appln. KOKAI Publication No. 11-297192. When it comes to the manufacturing process of the surface conduction device, an electrically discontinuous portion is formed in a thin film by the process called “forming”, followed by depositing a carbon-containing material on the thin film by the process called “activation” as in the manufacturing process of the planar MIM device described above. Compared with the planar MIM device described above, which exhibits the VCNR current-voltage characteristics, the surface conduction device disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-297192 referred to previously exhibits monotonously increasing current-voltage characteristics as shown in FIG. 3 in place of the VCNR characteristics. Also, the emission efficiency of the surface conduction device is about 10−3, which is higher than that for the planar MIM device.
A thin type planar display utilizing the surface conduction device described above exhibits nonlinear current-voltage characteristics as shown in FIG. 3 and, thus, it is possible to obtain a sufficient dynamic range with about three figures at a voltage amplitude of about 4 V to about 5 V. For example, it suffices to change the voltage applied to one of the electrodes of the device within a range of between 0 V and +5 V with a voltage of −5 V kept applied to the other electrode.
However, in the case of performing such a control, the current leakage takes place during the nonselection, i.e., when the minimum potential difference is provided between the electrodes. It is ideal for the leakage current to be as close to zero as possible in view of the power consumption and the load on the driver IC. However, the current leakage is not necessarily suppressed sufficiently at the present stage.